Silica pigments



Patented Sept. 14, 1937 UNITED STATES SILICA PIGMENTS Joseph W. Ayers, Easton, Pa... assignor to C. K. Williams & 00., Easton, Pa., a. corporation of Pennsylvania N Drawing. Application September 26, 1934, Serial No. 745,523

6 Claims.

This invention relates to silica products particularly suitable as pigments for use in paints, varnishes, lacquers and the like, and to processes of producing the same. The products are composed of altered naturally occurring silica, rather than of artificially prepared silica, as will be hereinafter more definitely described.

For many years there has been a demand in industry for a low priced white filling and extending pigment. Many compounds have been proposed for this purpose and some have been employed, such for instance as asbestine, china clay and barytes, which compounds have not been satisfactory either because of poor hiding power, poor suspension properties, high cost of raw material or of preparation, or any two or more of these deficiencies.

Silica even in the finest grades of so-called amorphous silica has heretofore not proven satisfactory as a pigment ingredient for paints because (1) it is highly abrasive and therefore destructive to the rolls used for grinding pigments into paints, (2) it usually contains discoloring impurities and (3) its specific gravity interferes with its ready suspension in normal paint vehicles.

The principal object of the present invention is to provide a silica product having all the necessary properties for a filling and extending pigment and from which the detrimental properties have been removed.

To accomplish this result I produce a treated silica product from a soft silica rock of microcrystalline structure such as that found in large deposits in southern Illinois, sometimes erroneously spoken of as amorphous silica.

As described in geological reports, this silica, referred to generally as Illinois soft silica, is a chalcedonic silica produced by the slow weathering of chalcedony. It is of a micro-crystalline- 0 fibrous structure, contains small quantities of a number of impurities including iron compounds and has a specific gravity of about 2.65. To the unaided eye it appears to be amorphous in character, but in its natural condition it is truly crystalline.

The novel silica product of the present invention is produced from the soft silica rock of the nature above described briefiy by grinding to a 50 desired particle size, washing to remove soluble impurities and bleaching and calcining for a period and at a temperature adapted to bring about the desired changes as will hereinafter be pointed out. This treatment has a marked effect upon the silica particularly in decreasing its specific gravity and in increasing its opacity or index of refraction.

In accordance with a detailed preferred embodiment of the process for the production of the silica suitable as a pigment, the Illinois soft silica 5 after being mined is crushed and then ground to the desired size preferably in a ball mill in the presence of water after which it is lixiviated, settled, filtered and dried. The dried product although of the desired fineness is still crystalline and has the same specific gravity as the original, namely 2.65. To the dried product, I then add a bleaching agent, for example, ammonium chloride, preferably in a ratio of about one-quarter to five per cent. by weight of the silica to be treated, 1 after which the mixture is calcined or heated at temperatures suitably of from about 1000 .to 1800 F. for a period of from about fifteen minutes to twohours, depending upon the amount of iron present and the degree of bleaching desired. At Q no time during the calcining operation is the silica sintered as this is to be avoided because of the formation of objectionable agglomerations which would necessitate further grinding.

The effect of this combined bleaching and cali cining operation on the silica particles is manifold. During the calcining operation the crystalline character of the particles is changed to an amorphous character. The specific gravity of the material is reduced from 2.65 to 2.32 or about 11%, the oil absorption is increased from 25.3% to 28.2%, the refractive index is changed from 1.56 to 1.55 and the percent reflecting power as indicated by spectrophotometric curves is increased from 91% to 96% or over through the wave lengths of 400 to 700 millimicrons.

The decrease in specific gravity gives greater Volume loading and the increase in fiufliness reduces the abrasiveness and provides better suspension properties in oil. The. product prepared 40 in this manner is suitable for pigment purposes, without further treatment.

The equipment used in effecting the above process does not constitute part of the present invention. However, certain types of apparatus have proved quite valuable from the commercial point of view. For example, the calcination has been most eifectively carried out in a continuously rotating kiln of a construction already known in the art.

Instead of ammonium chloride, other inorganic salt bleaching agents may be employed with more or less success. Ammonium chloride is preferred because it is completely volatilized during calcination and thus does not leave a diluent in in the form of a solution to the wet cake obtained i from the filter press and the material-calcined directly, thereby saving the cost of one drying operation. The bleaching effect, however, -is

somewhat better when the bleaching agent is added to the dried silica in dry form. When the silica product is to be usedrasanaddition to white enamels, etc., containing other-alwhite opaque pigments it has been found that a grinding operation after the calcination considerably "improves the texture ,ofthe, pigment.

The process of the present invention increases the amorphous characteristics of the silica to a zsubstantial..degree. .For example the specific gravityU-islreduced from 2.65 to about 2.32 or between-2320 and-2.350 which change increases the 'bulkiness lofithe'productand alsoincreases its ability to remain suspended in the liquid componentsisoflpaintsland the like. A powdered mass pr0duced. from thelherein described raw product .without employing .the bleaching and calcining =operations has; poor hiding power and very poor suspensioncproperties. I

v By the present process .thelreflective power or twhiteness ofthe silica is increased on the average of about 5 This change greatly increases'the'value of the product because of its increased .hiding power. Abrasive properties which are sosobjectionableiin the manufacture- 40-01 certain white paints such, for example, as those employed-on steel roller mills, are destroyed toasubstantial degree by the herein described treatment.

The productaof the present invention should '45 not .be; confused with .kieselguhr products which are essentiallyporousdiatomaceous earth prod- -uctsgproduced'fonuse: in.theafiltration and purification of various-fluids byv absorptiomor adsorption. Silicalon. the otherrhandrin itslnat- --;ural. stateis a crystalline non-porous abrasive in such enamels.

While the process .of the present invention has been described in detail as to the preferred steps, it should be understood that various changes may be made therein and the beneficial product herein described maybe produced by other steps coming within my invention .and the scope of the claims hereto. attached.

I claim:

1. A process of producing a pulverized, amen phous, white silica pigment free of agglomerations and having lower specific gravity, higher reflecting power, better suspension properties, a 5 lower refractive index and less abrasiveness than micro-crystalline soft silica, comprising grinding micro-crystalline soft silica, lixiviating and drying the pulverized mass, adding a small quantity of ammonium chloride, and heating the mass 10 to a temperature of from about 10001800 F.

2. A filling and extending pigment for paints, enamels, varnishes and the like consisting of an amorphous solf silica powder having a specific gravity at least 10% lower and a lower refractive 15 index and a reflecting power substantially 5% orrmore ,higher' and a higher oil absorption property as compared with natural chalcedonic silica and produced by calcining said natural chalcedonic silica, which has been pulverized and 20 lixiviated, in@ the presence of l a bleaching .agent at temperatures between 1000 and 1800-F.

3. A;process of producing a pulverized, amorphous, white silica pigment free of agglomerations and having lower specific gravity, a re- 25 -fiecting power substantially 5% or 'morelhigher,

dating and drying thepulverized mass, adding s30 a small quantity of an inorganic chloride: salt for effecting removal of color, and heatingthe massto atemperature of from about 1000 to 1800 F.

4. A process'of producing a pulverized,w amor- 3 phous, white silica pigment free of agglomerations and having lowerspecific gravity, higher re- .fiecting power, better suspensionproperties, a

lower refractive index; and less abrasiveness than microcrystalline soft, silica comprising grindingmo 'micro-crystalline soft .silica, adding .25 to -5% byweight 10f ammonium "chloride .thereto l and heating the;mass to atemperature of fromabout 1000 to 1800; F. i

5. A filling and extendingpigmentforpaints, enamels, varnishes and the like consistingyoian amorphous soft-silica ,powderphaving a specific gravity-at least 10% lower and alower refractive index and areflecting 5 power substantially 5% or more higher and ahigher oil absorption propn5 erty as-compared with natural chalcedonic silica andproduced by calcining saidnatural chalcedonic silica, "which has been;.-pu1verized, in the ,presence of a bleaching agentv at temperatures between 1000 and;1800 F.

ts ,6. A filling and extending pigment for;paints,

' enamelswvarnishes andvthe like consistingqofan amorphous soft silica powder having a. specific gravity ,at least 10% lower and a lower refractive index and a reflecting powersubstantiallyiis 5% or more higher and a higher oilflabsorption property as compared with natural chalcedonic 

